The present invention relates to a display device. In particular, the present invention relates to a display device adopting an active-matrix method.
A liquid-crystal display device is generally a liquid-crystal display device adopting an active-matrix method, which has a thin-film transistor (hereinafter, also abbreviated as “TFT”) functioning as a pixel transistor, for each of pixels which are laid out to form a matrix. The pixel TFT holds display electric charge when put in an open state or an electrically disconnected state.
To use a mobile phone or a smart phone for a long period of time, the phone is required to have a liquid-crystal panel consuming a small amount of power. An ordinary liquid-crystal panel carries out write operations at a frequency of 60 times per second. The amount of power consumed for driving the write operations cannot be ignored. In operations to display mails, web pages or the like, the displays appearing on the panel are mostly static displays. Thus, the write operations do not have to be carried out at a frequency of 60 times per second. Therefore, the panel is different from for example a TV for displaying moving images. In addition, it is possible to assume a device in which the display mode is switched from a moving image mode to a static image mode and vice versa. The moving image mode is a mode for displaying 60 comas per second whereas the static image mode is a mode for carrying out intermittent driving and holding the previous image in the mean time. To reduce the driving power, reducing the driving frequency, e.g. carrying out a writing operation only once per second, is effective. However, the human eye has an unfavorable sensitivity for luminance changes of a low frequency. Thus, if the luminance is not prevented from changing before and after the write operation, flickering is generated, causing the user to feel discomfort. In general, the human eye is not capable of recognizing flickering of a frequency of 60 Hz or higher. Thus, there is no problem even if the luminance changes. For a frequency lower than 60 Hz, however, the luminance changes must be kept at a value smaller than a recognition limit.
As described above, in the case of an active-matrix liquid-crystal display device, a TFT is provided for each of pixels laid out to form a matrix and, in a post-write period for holding data, pixel electrode charge is held by putting the TFTs in an open state or an electrically disconnected state. If the off characteristic of the pixel TFT is poor, the electric charge leaks out during the data holding period. Thus, the voltage after the data holding period is different from the initial value so that the luminance changes. As a result, there appears a phenomenon in which the luminance changes when a write operation is carried out again. The off characteristic, showing the length of a period in which electric charge can be held reliably during the data holding period, is an important parameter.
In addition, in recent years, there has also been announced an active-matrix liquid-crystal display device made by using an oxide semiconductor to provide a small off current as a characteristic. In general, however, the TFT of a high-precision active-matrix liquid-crystal display device such as that used in a smart phone is a low-temperature poly-silicon TFT in many cases. This is because there are merits that the size of the TFT can be reduced and a logic circuit such as a scanning circuit can be created. Also in the future, the TFT of a high-precision active-matrix liquid-crystal display device is a low-temperature poly-silicon TFT in many cases. According to studies made by inventors of the present invention, however, if the intermittent driving is carried out by using this low-temperature poly-silicon TFT, there are raised problems that the luminance changes are big and easily recognized as display flickers.